Transmitting circuit



Sept. 13, 1932. ,.L AV s 1,877,561

TRANSMITTING CIRCUIT Filed Sept. 13, 1928 2 Sheets-Sheet 1 WIN a, mm

[III I]: llllli INVENTOR Rob err Lflavis.

ATTbRNEY Sept; 13, 1932. R. 1.. DAVIS 1,877,561

TRANSMITTING CIRCUIT Filed Sept. 15, 1928 2 Sheets-Sheet 2 nun tII'IIIII T Q T 'Q Detector "l INVENTOR' R m Robert L.Davi.s.

, I Y E ATTORNEY as having Patented Sept. 13, 1932 UNITED STATES PATENToF Ic ROBERT L. DAVIS, 0F PITTSBURGH, PENNSYLVANIA, ASSIGNOR I'OWESTINGHOUSE ELECTRIC & MANUFACTURING" COMPANY, A CORPORATION OFPENNSYLVANIA TRANSMITTING GIRCUIT I Application filed September 13,1928. Serial No. 305,?22.

- This invention relates to radio communication and more particularly toa multiplex system whereby several messages may be sent simultaneouslyover one system.

It has been frequently proposed to transmit from the sending station aplurality of carrier. frequencies, each modulated in accordance with itsown particular message. The messages may then be received either atseveral different stations or at the same station by means of aplurality of receiving sets, each tuned to its particular carrierfrequency.

It has also been proposed to modulate the amplitude of a carrier currentof constant frequency with. several modulations each having its owncharacteristic frequency. The several modulations, being appliedin turn,a plurality of messages may be sent over the one communication channel.I

It has also been proposed to modulatea carrier frequency by changing nottheamplitude but the frequency thereof. By my invention, I propose toapply the latter method of modulation to a single carrier frequencyandthereby send a plurality of messages over what is, in effect, asingle radio channel. j

The method of modulation by changing the frequency differs from thechange of frequency made in order to tune the sending set to one oranother of several receiving stations in two respects. (1) The changesin frequency are much smaller than the changes used to accomplish saidtuning, and (2) they are not great enough to cause the modifiedfrequency to fall outside the range to which the receiving set issensitive. v The small changes of frequency used for frequency-shiftmodulation are produced by changing the conditions under which anoscillation generator, of a type which produces a nearly constantfrequency operates.

The crystal-controlled vacuum tube and the harmonicgenerator governed bya'tuning fork arethe best known examples of such generators. They wereoriginally described study of them has shown that they change theirfrequencies slightly in response to large changes in certain conditions.This property a constant frequency but further.

has been used fo producing frequency-shift modulation. r

It is an object of my invention to employ a plurality of differentfrequency shift-modulations upon 7 mit a plurality of different messagessimultaneously over one radio channel. Y

It is a further object of my invention to distinguish with rapidity andcertainty between the slightly differing frequencies produced byfrequency-shift modulation-of one carrier frequency.

It is a further object of my invention to provide a' plurality oftranslating devices in one receiving set and to provide means wherebyeach of the several nearly equal frequencies resulting from theabovedescribed frequency-shift modulation shall control one of. saidtranslating devices.

providea plurality of signal-translating devices and to so arrange theabove-mentioned frequency-responsive. translating devices that theoperation of one of them will actuate all of said signal-translatingdevices in one sense and the actuation of anyv of the others willoperate the respective signaLtransIating device in the opposite sense;

It is a further object of my invention to provide means wherebyintermittent production of any one of said nearly equal frequene ciesduring a period constituting an element of thesignal shall produce thesameefl'ect in the final translation of the signal or a continuousproduction of said frequency during said period. T

Other objects .ofthe invention and details of the constructionwill bereadily understood from the following description and the accompanyingdrawings, in which:

Figure 1 is a diagram of the circuits and apparatus at the sendingstation, and

ig. 2 is a similar diagram for the receiving station. 7

In Fig. 1, the vacuumtube 1 has associated with it the usual circuits toconstitute an oscillation generator, the frequency of which is fixed bythe piezo-electriccrystal'2. These circuits'include an inductor 3shunted across the crystal. Across a portion of the inductor 3, vacuumtube 4 is connected. The connection includes a B-battery 5, and bothtube and battery are connected to the inductor through a condenser 6 andthe usual C-batter 7.

lit has been discovered that, with such arrangement, the frequency ofthe oscillations generated by the tube 1 will change somewhat inresponse to changes of the potential upon the grid of the vacuum tube 4.To provide these potential changes, I connect the grid through aresistor 10, and a small portion of the battery 11 to the filament.Additional portions of the batery 11 areconnected to the grid throughkey-controlled devices.

The keys 12, 13, and 14 are connected, respectively, to points of thebattery 11, including successively larger portions thereof. The key 12is connected, through a slip ring 15, to a segment 16 of a commutatorwheel 17. Similarly, the key 13 is connected, through a slip ring 18, toa segment 19, and the key 14 is connected, through a slip ring 21, to asegment 22. The segments of the 4 wheel 17 are insulated from each otherand from the central portion of the wheel. The slip rings and wheel aremounted upon a shaft 23 which is intended to represent any suitabledriving means. The shaft is driven at such speed that all of thesegments will be engaged by the brush'24 several times during theshortest time that the key remains either open or the device. The brush24 is connected to the resistor 10 and the grid of the tube 4.

The output circuit of the oscillation generator 1 supplies energy to theamplifier which delivers energy to the antenna 27 Instead of an antenna,the device may, if preferred, deliver its energy to a line forcarrier-current communication. Instead of a simple amplifier, the devicemay, if desired, include a frequency multiplier.

In the receiving system illustrated in- Fig. 2, an antenna 31 is shown,which, if preferred, may be replaced by any of the usual devices forreceiving high frequency energy from a carrier-current line. Theconnections from the antenna 31 to the detector 32 include the secondary33 of a transformer through which a frequency differing slightly fromthe carrier frequency is introduced. This heterodyne frequency isprovided from any suitable source, such as the oscillation generator 34.The connections from the antenna. 31 tothe detector 32 may include anyof the usual or known features of a hetero-- dyne receiver which it isdesired to employ.

The output of the detector 32 is delivered to a plurality offrequency-selective.devices such as filters, tuned circuits or the likearranged in parallel. The first of these (35) is series resonant to. thebeat frequency which arisesbetween the energy received closed in normaloperation of from the antenna 27 and the energy from the generator 34when all of the keys 12, 13 and 14 are open. The next (36) is seriesresonant to the beat frequency between the energy from the generator 34and that from the antenna when the key 12 is closed and the rush 24 ison the segment 16. The next (37) is series resonant to the beatfrequency which corresponds to conditions when the key 13 is closed andthe brush 24 is on the segment 19. The next (38) is series resonant tothe beat frequency when the key 14 is closed and the brush 24 is on thesegment 22.

From the point of greatest potential change in a series-resonantcircuit, a connection is made, over a condenser, to the grid of a vacuumtube. Thus, from the junction between the inductor and the condenser inthe first tuned circuit 35, over the condenser 41, a connection is madeto the grid of a vacuum tube 42. The grid of-this tube is connected tothe filament over a grid leak 43, and the plate circuit of the tube 42includes the winding of a relay 44 and a B-battery 45.

The tuned circuit 36 is similarly associated.

with a vacuum tube 46, the plate circuit of which includes the windingof a slow-acting relay 47 and the B-battery 45. The tuned circuit 37 isassociated in the same way with a vacuum tube 48 controlling aslow-acting re lay 49, and the tuned circuit 38 is associated with avacuum tube 51 controlling a slowacting relay 52. v I

The contacts of the relay 44 .are connected in series with theleft-hand'windings of three differential relays 54, 55 and 56 and tobattery 57. This series arrangement may, if desired, also include arheostat 58 for adjusting the forces exerted by said left-hand windings.

The contacts of relays 47, 49 and 52 are connected in parallel, theparallel circuits being energized from a common battery 61 andincluding, respectively, the right-handwindings of the relays 54, 55 and56. Each, of these relays actuates any desirable signalling device. Forthe purpose of illustration, these are shown asrecorders, whereby themessages are marked upon moving tapes.

In'the operation of the device, the oscillation generator 1 produces afrequency which is nearly constant. Small changes in this frequencyoccur as described below, but the changes are a very small percentage ofthe normal frequency which has been referred to herein as the carrierfrequency. The carrier frequency is amplified in the amplifier. anddelivered to a line or to space over the antenna 27. u

WVhenall the keys 12, 13 and 14 are open, the frequency thus deliveredis the carrier frequency, regardless of the positionof the commutatorwheel 17. This may, instead of being the frequency generated by the tube1, be a multiple. thereof, if the amplifier. is

made to deliver some harmonic instead of the fundamental frequency.

The carrier frequency, is received on the antenna 31 and produces, withthe energy from the generator 34, a beat note which, amplified, ifdesired, is delivered by the detector 32. This frequency produces littleeffect upon the tubes 46, 48 and 51 because the tuned circuits 36, 37and 38 are not resonant to it. The tuned circuit 35, being resonant tothis frequency, impresses a larger potential of this frequency upon thecondenser 41 and, thus, upon the grid of the tube 42. V

The tube 42, under these circumstances, will deliver a much smallerplate current than when oscillations are not impressed upon the gridthereof. Consequently, the relay 44 releases its armature, and thecircuit through all of the left-hand windings of the relays 54, 55 and56 is energized. If, at this moment, the right-hand winding of any ofthese relays is de-energized, the corresponding indi eating device willbe moved by the left-hand winding.

When key 12 in Fig. 1 is closed, the potential upon the grid ofthe tube4, instead of corresponding, all of the time, to the portion of thebattery 11 between the filament and the resistor 10, will, at times,correspond to the potential of that portion of the battery between thefilament and the connector to the key 12. The latter correspondenceoccurs each time that the sector 16 comes into contact with the brush23.

Thus, if the shaft 24 is rotating fifty times per second and theshortest time that the key 12 is closed in normal operation is a fifthof a second, there will be at least ten intervals during which thefrequency delivered from the antenna 27, instead of being the normalcarrier frequency, differs therefrom by an amount corresponding tothepotential controlled by the key 12.

The shortest time that the key is closed, or open, is a periodconstituting the signal element. The signal will be made up of combinations of this period with multiples thereof grouped together to formthe code characters.

When the key 12 is closed for a longer time, there will be more than tensuch intervals during its closure, but, regardless of the number ofintervals during which the frequency, characteristic of the key 12, isradiated, the total time which such radiations endure will be one-thirdof the time the key 12 is closed. During the other two-thirds of thetime, the brush 24 is not in contact with the segment 16 and the key 12is, therefore, without effect upon the radiated frequency.

During each of the intervals that the frequency of the radiationscorresponds to the potential controlled by the key 12, the radiationsreceived at the antenna 31 will combine with the energy from thegenerator 34 to ed through the tuned circuit35 but is readily'conductedthroughthe tuned circuit 36. This produces a' diminution inplate currentfrom producefln the output of the detector 32, a I beatfrequencywhich is not readily conductbeat frequency is therefore,without effect upon-the tube 42 or'the tubes 48 and 51 but the tube'46and thus a diminution in the cur- 7 rent in the winding of the relay 47.The slow-acting character of this relay preventsithe' armature frommoving in reis, therefore, closed as long, or'substantially as long, asthe key v12 is closed.

The left-hand winding of theclrelay 54 is energized by current from thebattery .57 dure ing not more than two-thirds-ofthe time that the key 12is closed. Itwill be quite twotllildS, if, and only if, the keys13 and.14 remain open allrthe while that the key 12 is closed. The left-handwinding of the relay V 54 is so designed and the rheostat. 58 is soadjusted that it is only when the currentfrom the battery 57 is presentin the left-hand winding all of the time that it is able tocounteractthecurrent in the right-hand winding. Consequently, the armature of therelay 54 'moves in the direction corresponding to the right hand windingso long as the key 12 is closed. j j l Similarly, the relay 55 willbeunder the.infiuence of its; right-handwinding so long as thekey 13 isclosed, and the relay 56 will be under the infiuence of its right-handwinding so long as the key 14 is closed. j "Closure of the key 131swithout effect upon the relay .54 because it does not influence themovement of the armature of relay 47 and its effect upon the movement ofthe armature of relay 44 is only to diminish by another third thetime(already diminished by onethird) during which there is current in theleft-hand winding.

It will be apparent from the foregoing that the relay 44 is the only onewhich must act as rapidly as the segments of the wheel 17 2 pass thebrush 23. The relays 47 49 and 52 need be no more rapid than the keyingfreltO quency, and'the relays 54, 55 and 56 need be no more rapid thanthe keying frequency. If it is desired to operate wheel 17 at a higher aspeed than that to which the armature movements of a relay of the typeshown at 44 can correspond, a relay of the vacuum tube type may besubstituted for the relay 44without difficulty. 7

Because the several messages are conveyed by frequency changes insteadof amplitude .modulation, it is possible to receivethem by a heterodyne'instead of an ordinary detector. The greater sensitiveness of theheterodyne method of reception is then combined with multiplexcommunication.

Other changes in details of the construction will occur to those skilledin the art without specific mention thereof. The description of only onespecific form of the apparatus is not to be construed as a limitation.No limitation is intended except as indicated in the accompanying claimsor required by the prior art.

- I claim as my invention:

1. In a radio multiplex transmitting system, a crystal-controlledoscillation generator, circuits associated with said generator includinga variable impedance for changing the frequency of the oscillationsgenerated thereby, a plurality of separately controlled means forbringing said impedance to respectively predetermined values and cyclicmeans for operatively relating said impedance to each of'said separatelycontrolled means in turn, the speed of .said cyclic means beingsufficient to operatively relate the impedance to any one of saidseparately controlled means a plurality of times during the normalperiod of action, of the separately controlled means. 2. In a radiomultiplex system, a crystalcontrolled oscillation generator, a pluralityof keys, means controlled by said keys for causing said oscillationgenerator'to produce frequencies which differ from the frequencygenerated thereby when all said keys are in inoperative position byamounts respectively predetermined for each key, a commutator connectingeach key in turn to said means, said commutator being driven at a speedsufficient to connect each key to said means a plurality of times duringone period of normal keying frequency and a transmitter controlled bysaid generator. I In testimony whereof, I have hereunto subscribed myname this 6th day of September ROBERT L. JDAVIS;

